the may 10, 2007 issue of the new england journal of medicine describes two large randomized, placebo-controlled trials of a vaccine for hpv-related cancer. hpv is human papillomavirus, the most common form of sexually transmitted infection. for the group of people who received the vaccine, the hpv-related cancer rate was much lower than for those who received a placebo.this study is an example of an --study. the response variable is